Molecular phylogenetic studies have found that Hernandiaceae, Lauraceae, and Monimiaceae sensu stricto form a monophyletic group, here referred to as the HLM clade. Due to a paucity of phylogenetically informative substitutions relationships among the three families have remained unclear. We analyzed a total of 2,808 aligned nucleotides from a plastid intron, three spacers, and a portion of the nuclear 26S rDNA gene in a dense sample of HLM genera, using four outgroups with differing substitution rates. Despite obtaining single best topologies with maximum likelihood, minimum evolution, and parsimony approaches, family relationships remained as poorly supported as in the previous molecular studies. Exploration of the data indicates that varying substitution rates across lineages or sites, insufficient taxon sampling, fast evolving outgroups, or biased base composition are unlikely to explain the difficult reconstruction. Exclusion of the longest-branched taxa, such as the hemiparasite Cassytha, which is nested in Lauraceae, certain Hernandiaceae, and outgroups, had no effect on family relationships. To resolve HLM relationships one could add some of the basal genera sequenced here to the existing five-gene data set of Qiu et al. (1999) to increase taxon sampling density. Alternatively, and this is the strategy we will pursue, one can sequence low-copy nuclear genes for key HLM genera to sample "faster" genes.

Key words: Cassytha, Hernandiaceae, Lauraceae, Monimiaceae, substitution rate variation